Solvolysis of the Quinoline 5,6- and 7,8-Oxides: Effect of the Ring Nitrogen

Article abstract of DOI:10.1021/ja00189a051

pH-rate profiles for the hydrolysis of quinoline arene oxides were measured in 1:9 dioxane-water at 25 deg C (0.1 M NaClO4) and compared with the parent carbocylic compound naphthalene 1,2-oxide.Unlike naphthalene 1,2-oxide whose hydrolysis rate shows a first-order dependence on hydronium ion concentration (k_H) below pH 5, rates for the quinoline 5,6- and 7,8-oxides show a k_H reaction (0.14 and 1.54 M^-1 s^-1 for quinoline 5,6- and 7,8-oxides, respectively), but plateau below pH 2-3. pH-independent rate constants in the low-pH plateau are 2.7E-5 and 1.7E-3 s^-1for the 5,6- and 7,8-oxides, respectively.On the basis of studies of the N-methyl cation of the 5,6-oxide (k_obsd < 4E-7 s^-1 at pH 1.8), it is concluded that the low-pH plateau is due to lack of reactivity for the N-protonated quinoline oxides.Also unlike naphthalene oxide, the quinoline oxides show a reaction with hydroxide ion (k_OH rate) that results in the formation of trans dihydrodiols.In acid, quinoline 5,6-oxide, but not quinoline 7,8-oxide, gives a significant yield (20percent) of the trans dihydrodiol product and provides the first example of a benzo-ring arene oxide that forms dihydrodiol product under acidic conditions.Other products of the acid-catalyzed reaction of the 5,6-oxide are 5- (73percent) and 6- (7percent) hydroxyquinolines.The 7,8-oxide upon solvolysis in acid gave exclusively 8-hydroxyquinoline.The 100-1000 fold decrease in reactivity of the quinoline oxides, relative to naphthalene oxide in the pH range of 1-10, can be accounted for in terms of the presence of their ring nitrogens and the differences in the pK_a values of these nitrogens.